A magazine news story on the unification of physics usually begins by saying that Einstein’s general theory of relativity and quantum theory are irreconcilable. The one handles the force of gravity, the other takes care of electromagnetic and nuclear forces, but neither covers all, so physicists are left with a big jagged crack running down the middle of their theoretical world. It’s a nice story line, except it’s not true. “Everyone says quantum mechanics and gravity don’t get along—they’re incompatible,” says John F. Donoghue of the University of Massachusetts at Amherst. “And you still hear that, but it’s wrong.”
The famous physicist Richard Feynman came up with a seamless quantum theory of gravity in the 1960s. It looks much like the quantum theories of the other forces. Just as photons convey the force of electromagnetism, particles called gravitons convey the force of gravity. Where the forces differ is that electromagnetism behaves in essentially the same simple way on all scales, varying only in its general strength, whereas gravity becomes increasingly rococo as you zoom into microscopic scales—signaling that the theory eventually gives way to a deeper one such as string theory or loop quantum gravity. But “eventually” is so far off that physicists can usually neglect the rocococity. In the 1990s Donoghue and others began to use Feynman’s theory as a working, or “effective,” theory; though not the final word, it closes up the crack between gravity and the other forces on medium to large scales.
In 2006 Sean P. Robinson and Frank Wilczek of the Massachusetts Institute of Technology applied the effective theory to see whether gravity changes the way forces vary in strength with scale. If gravity doesn't interfere, electromagnetism should become equal in strength to the nuclear forces at one scale and to gravity at a different scale. Robinson and Wilczek conjectured that gravity saps the strength of the other forces and causes them all to match up at the same scale. The idea didn’t pan out but did get people thinking about how the forces mess with one another.
Last November, David J. Toms of Newcastle University in England argued that even if gravity does not bring all the forces into line, it at least qualitatively reconciles electromagnetism with the nuclear forces. Neglecting gravity, electromagnetism intensifies as you go down in size, whereas the nuclear forces weaken. But gravity emasculates electromagnetism, causing it to behave like the nuclear forces on the very smallest scales.
Wilczek calls Toms’s paper “impressive.” Around the same time, however, Donoghue and his graduate students Mohamed M. Anber and Mohamed El-Houssieny cast doubt on the whole approach. Although gravity surely interferes with the other forces in some way or other, they question whether the effect is so straightforward as a tweak to the force strength. The rocococity of gravity should infect the other forces.
One reason physicists can reach such diametrically opposite conclusions is that the calculations are complicated and no one yet knows how to interpret them. “I really wish I had a physical understanding of what is going on, and I don’t,” Toms says. To paraphrase Ernest Rutherford, discoverer of the atomic nucleus, physicists don’t consider they have understood something unless they can explain it in plain language to a bartender. Fortunately for quantum gravity theorists, the bartenders of the world are a patient group.